Fortunately, most of the links from my site are to the Oracle docs, whose URLs haven’t changed, but there are also plenty of OTN links. I’m trying to clean up the problem links, but it’s going to take a little time. If you spot any broken links, or links that don’t look like they point to the intended information, feel free to contact me and I’ll do my best to sort them.

I think anyone who has read Wolfgang Breitling’s material about the optimizer will be familiar with the concept of Cardinality Feedback and one particular detail that when Oracle gets a cardinality estimate of one for a “driving” table then there’s a good chance that the execution plan will go wrong. (That’s not rule, by the way, just a fairly common observation after things have gone wrong.)

A recent note on OTN reminded me of a particular scenario where this specific problem can occur. It’s not particularly common, but it may hit people who are building data warehouses from multiple different sources. We start with an unlikely looking data set and very simple query:

What do you think Oracle estimated cardinality will be for this predciate ? We know, because we saw the data being built, that we’re going to identify 10,000 rows. But the optimizer doesn’t see it that way – check line 2 of the execution plan. The optimizer thinks it will find just one row:

One of my Yoga buddies was given a laptop by is dad and wanted to get it connected over wireless. His dad also gave him a wireless ADSL router, but couldn’t get it set up. This sounds like a job for Captain Support…

The router wasn’t able to connect to the internet. It turned out that the router was not working properly and needed a firmware update. Next issue was the wireless connection between the router and the laptop was kinda funky. The connection would never work when any form of encryption was turned on. In the end I had to turn off encryption and stopped the router from broadcasting in an attempt to reduce the chances of people piggy-backing on it.

How are normal folk meant to cope with this? The answer is they don’t and they need Captain Support…